The subject matter disclosed herein relates to gas stream sampling and, in particular, to reducing the number of particulates from entering a sample probe at the sampling location of a gas stream.
Fly ash is one of several pollutant particulate residues generated in the combustion of coal or other fossil fuels by, e.g., boilers or furnaces. Fly ash is generally captured from the chimneys of coal-fired power plants. In the past, fly ash was generally released into the atmosphere, but pollution control equipment mandated in recent decades now requires that the fly ash be captured instead of being released into the environment. In the U.S., the fly ash is now generally collected and stored at the power plant. Depending upon the source and makeup of the coal being burned, the components of the fly ash produced vary considerably, but fly ash typically includes substantial amounts of silicon dioxide (SiO2) (both amorphous and crystalline) and calcium oxide (CaO). Fly ash is commonly used to supplement cement in concrete production, where it can bring both technological and economic benefits, and is increasingly finding use in the synthesis of geopolymers and zeolites.
However, when sampling a gas stream, for example, in a combustion furnace or boiler operating at relatively high temperatures (e.g., 900° F.-1500° F.), it is generally difficult to continuously separate on-line or in-situ the relatively hot fly ash from the sampling flue gas such that primarily the flue gas is sensed. Removal of fly ash in a relatively substantial amount from the gas sampling stream at or near the sample probe or sensor is needed for typical sampling applications. Failure to reduce the amount of fly ash leads to: 1) ash accumulation within the sampling probe, which may lead to plugging of the probe; and 2) ash accumulation on the analyzer's sensors, which may reduce or impair the sensing ability and accuracy and also the lifetime of the sensors.
There exist many techniques to remove fly ash from hot flue gas sampling streams. A common technique is to use a sampling conditioning system to cool down the temperature of the sample flue gas. The separation of fly ash from the sample flue gas stream is then performed via a filtering device such as a fabric filter, cyclone, or other filtering device system. This approach turns out to be cumbersome and expensive due to the additional parts needed. In addition, this type of system usually requires relatively high maintenance due to ash disposal requirements from the filtering system.